Biomass is an increasingly popular starting material for production of a variety of materials. Ever growing energy demands and environmental concerns have particularly prompted much toward work developing convenient and efficient pathways for converting biomass to biofuels, valuable chemicals, and biomaterials.
Wood is the most abundant lignocellulosic resource on the planet. Although wood has long been used as raw materials for building, fuel, and various products, its use for converting to biofuel and producing valuable chemicals and biomaterials has only recently been considered in light of development of bioengineering and catalytic chemistry.
The complex structure of wood makes it insoluble in common molecular solvents, and preliminary chemical or physical treatment is thus necessary for further applications. Such preliminary treatments, especially chemical treatment, are generally undesirable because of the need to use or release environmental unfriendly chemicals. For example, NaOH and NaSH typically must be used to delignify wood in the kraft pulping manufacturing technology, which is the most popular method used in the paper industry.
For the traditional conversion of wood into composite-materials, wood flour is used or heterogeneous chemical modification is performed. Performing these processes is plagued by feedstock-degradation, as well as the unavoidable consumption of large amounts of energy and expensive chemicals. The traditional method to obtain biodegradable plastic and composites is heterogeneous graft modification, which has been disclosed in U.S. Pat. Nos. 5,424,382, 5,741,875, 5,852,069, and 6,013,774. These methods suffer drawbacks such as low efficiency and utilization of hazardous chemicals.
Lignin is a vastly under-utilized natural polymer. Commercial lignin is currently produced as a co-product of the paper industry, separated from trees by a chemical pulping process. Lignosulfonates (also called lignin sulfonates and sulfite lignins) are products of sulfite pulping. Kraft lignins (also called sulfate lignins) are obtained from the Kraft pulping process. Other delignification technologies use an organic solvent or a high pressure steam treatment to remove lignins from plants. Because lignins are very complex natural polymers with many random couplings, the exact chemical structure is not known, and the physical and chemical properties of lignin can differ depending on the extraction technology and the plant material from which it is extracted. For example, lignosulfonates are hydrophilic and Kraft lignins are hydrophobic. Lignin is typically used as a stabilizer (e.g. an antioxidant) for plastics and rubber, as well as in the formulation of dispersants, adhesives, and surfactants. Lignin or lignin derivatives have also been used in the production of fully biodegradable lignin-based composites.
Ionic liquids have recently received much attention as “green” (environmentally friendly), designable solvents, which are favorable in light of the growing realization of the need to protect the environment. Ionic liquids represent a new way of thinking with regard to solvents. The field is experiencing rapid growth, and offers a starting point for science, industry, and business to cooperate in the formation of a new paradigm of green chemistry and sustainable industry.
Ionic liquids offer a range of significant improvements upon conventional solvents, and also exhibit greater ability than water for solubilizing organic compounds. The unique structure of ionic liquids compared to traditional molecular solvents provides for many unique solubilization characteristics. For example, a range of ionic liquids applicable for the dissolution of cellulose are disclosed in U.S. Pat. No. 6,824,559. Furthermore, ionic liquids have shown good solubility characteristics for monomers or polymers and have been used to reconstitute advanced composites materials, as disclosed in International Publication WO 2005/098546.
Ethanol, also known as grain alcohol, is presently made primarily from the starches and sugars in kernels of field corn. However, starches and sugars constitute only a small portion of plant matter generally. It has heretofore been impossible to employ starches to produce ethanol for biofuel use due to the limitation of available agricultural crops and excessive associated cost. Thus, the ability to commercially produce biofuels, such as bioethanol, is limited by the availability of a low cost, sustainable, and renewable feedstock. As forest resources are sustainably available on an annual basis, lignocellulosics offer an attractive feedstock; as previously noted, the economical and efficient use of such has heretofore been very limited.
Processing of lignocellulosics to ethanol consists of four major unit operations: pretreatment, hydrolysis, fermentation, and product separation/purification. Pretreatment is one of the most important operations for practical cellulose conversion processes, and is a key technical barrier to using cellulosic feedstocks for bioconversion. Pretreatment is required to alter the structure of cellulosic biomass to make cellulose more accessible to the enzymes that convert the carbohydrate polymers into fermentable sugars. An effective pretreatment will disrupt the physical and chemical barriers posed by cell walls, as well as cellulose crystallinity, so that hydrolytic enzymes can access the biomass macrostructure. The low accessibility of enzymes into untreated lignocellulosic matrices is the key hurdle to the commercial success of converting cellulosic biomass to biofuel.
Pretreatment has been viewed as one of the most expensive processing steps in cellulosic biomass-to-fermentable sugar conversion and has a major influence on the cost of most other operations. Effective pretreatment can significantly reduce the use of expensive enzymes. Moreover, pretreatment can strongly influence downstream costs by determining fermentation toxicity, enzymatic hydrolysis rates, enzyme loadings, mixing power, product concentrations, product purification, waste treatment demands, power generation, and other process variables. Of course, the pretreatment operation itself must be low in cost and energy consumption and avoid high consumption of expensive chemicals and feedstock degradation. Thus, there still remains a need in the art for methods of making a greater quantity of biomass readily available for efficient, low-cost conversion to biofuels.